In the past, monopole antennae and sleeve antennae have been considered as V2X antennae. In FIG. 22 to FIG. 26, the configuration, VSWR (Voltage Standing Wave Ratio), and gain in horizontal plane (X-Y plane) of a monopole antenna are shown, and in FIGS. 27A and 27B to FIG. 31, the configuration, VSWR, and gain in horizontal plane of a sleeve antenna are shown.
With reference to FIG. 22 to FIG. 26, a monopole antenna will be described. FIG. 22 shows the case where a monopole antenna 1 is installed vertically on a circular ground plate 5 (a circular conductor plate having a diameter of 1 m), and as shown in FIG. 23, the VSWR in this case is 1.4209 at a frequency of 5850 MHz, and is 1.4076 at a frequency of 5887.5 MHz, and is 1.4045 at a frequency of 5925 MHz. In the case where orthogonal coordinates X, Y, and Z are defined as shown in FIG. 22, the gain in horizontal plane of vertical polarized wave (Vertical Polarization) at 5850 MHz is shown in FIG. 24, and the average gain is −0.87 dBi. Also, the gain in horizontal plane of vertical polarized wave at 5887.5 MHz is shown in FIG. 25, and the average gain is −0.86 dBi, and the gain in horizontal plane of vertical polarized wave at 5925 MHz is shown in FIG. 26, and the average gain is −0.85 dBi. Like this, the monopole antenna (in the case where the monopole antenna is installed vertically on a circular ground plate having a diameter of 1 m, the average gain of vertical polarized wave is about −0.9 dBi) has the disadvantage that when the monopole antenna is installed on the roof of a vehicle body or the like, it cannot satisfy specifications required for V2X communication.
With reference to FIGS. 27A and 27B to FIG. 31, a sleeve antenna will be described. In the case where a sleeve antenna 2 shown in the front view of FIG. 27A and the cross-sectional view of FIG. 27B is installed vertically on a circular ground plate 5, as shown in FIG. 28, the VSWR is 1.0771 at the frequency of 5850 MHz, and is 1.0577 at the frequency of 5887.5 MHz, and is 1.0839 at the frequency of 5925 MHz. In the case where orthogonal coordinates X, Y. and Z are defined as shown in FIG. 27A, the gain in horizontal plane of vertical polarized wave at 5850 MHz is shown in FIG. 29, and the average gain is 2.27 dBi. Also, the gain in horizontal plane of vertical polarized wave at 5887.5 MHz is shown in FIG. 30, and the average gain is 2.35 dBi, and the gain in horizontal plane of vertical polarized wave at 5925 MHz is shown in FIG. 31, and the average gain is 2.38 dBi. Like this, the sleeve antenna has higher gain as compared to the monopole antenna, but has the disadvantage that mechanism design becomes difficult and the cost increases since it is required to three-dimensionally configure a coaxial structure and a sleeve structure with high accuracy.
By the way, Patent Document 1 mentions about a monopole antenna for V2X communication.